Closed loop fuel/air control may be enhanced in terms of response speed and accuracy by using a linear or a wide band continuous universal exhaust gas oxygen (UEGO) sensor versus a switching type exhaust gas oxygen (EGO) sensor.
However, the inventors have recognized several potential issues with such an approach. For example, closed loop fuel/air control using the UEGO sensor is still hindered by exhaust gas path dynamics. Specifically, a relatively large time delay (time between a fuel change and the first indication of a measured fuel/air ratio response) exists that destabilizes the closed loop fuel/air control, resulting in low gain feedback control with sluggish response speed. This limits the ability to properly regulate aggressive modulation of the exhaust feed gas which reduces catalyst efficiency. Moreover, it compromises the ability to facilitate disturbance rejection, making the control approach more vulnerable to conditions of reduced drivability.
The inventors herein have developed a closed loop fuel control system for an engine that compensates for the time delay to increase the response speed of the fuel control. For example, the system includes a reference input to produce a desired fuel/air signal, a delay compensation filter to receive a sum of the desired fuel/air signal and a fuel/air control signal output from a proportional-integral controller, the delay compensation filter providing a delay compensation signal, a filtered desired fuel/air signal used to calculate an error signal, an exhaust gas sensor to provide a fuel/air ratio signal that is subtracted from the filtered desired fuel/air signal and this result is added to the delay compensation signal to produce an error signal being provided to the proportional-integral controller to produce the fuel/air control signal, and a transient fuel control filter to adjust the fuel/air control signal according to an engine temperature dependent time constant and an engine temperature dependent gain to produce an engine temperature dependent delay compensated fuel/air control signal.
As an example, the delay compensation filter may be a Smith Predictor feedback control loop (Smith, O. J., “A controller to overcome dead-time,” ISA Journal, Volume 6, pg 28-33, 1959). The Smith Predictor feedback control loop includes a model that separately characterizes the time delay of the control system and the continuous time dynamics of the controlled system. The Smith Predictor feedback control loop can be modified to avoid interfering with the conventional fuel control system that makes feed forward adjustments based on reference changes due to, for example, varying driver's demand, yet still provide delay compensation to maintain stability of the closed loop system with high control gain. The conventional Smith Predictor and the modified version described here allow the controller to regulate the continuous dynamics of the system, only adjusting for delay when the measured signal differs from the Smith Predictor's estimate.
Furthermore, by feeding the delay compensated fuel/air control signal through the transient fuel control filter, the control signal may be adjusted based on engine temperature in order to compensate the effects of fuel puddle dynamics. In other words, as the rate of fuel evaporation in the intake ports of the engine vary with engine temperature, the fuel control signal can be adjusted to maintain accurate fuel control. In this way, accuracy of the fuel control response can be increased resulting in increased emissions control device efficiency and fuel economy. This closed loop adjustment for the fuel puddle dynamics is independent of and in addition to any conventional open loop transient fuel compensation adders that are a standard automotive control practice.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.